Engine components may degrade during operation in various ways. For example, an engine cylinder in an engine may start mis-firing due to a worn out ignition plug. One approach to detect engine degradation is to monitor engine speed. Diagnostic routines can monitor whether components of the engine speed rise above a threshold level and generate diagnostic codes or other indications requesting service, de-rating engine power, or shutting down the engine. However, the inventors herein have recognized that analysis of engine speed is often inadequate to thoroughly diagnose an engine problem.